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Potentiometer

Rockgirl

Aug 26, 2021
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Hi, I need help. Can I find any potentiometers where the rotation angle is smaller than usual? Usually it's I guess 290 degrees but can somewhere find about 90 degrees? I need a 100k potentiometer that adjusts the 90 degree rotation, I don't know if they are anywhere.
 

Harald Kapp

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Can you use a 330 kΩ potentiometer and limit the rotation mechanically to 90 °?
I'm not aware of 90 ° potentiometers off the shelf.
 

Rockgirl

Aug 26, 2021
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Can you use a 330 kΩ potentiometer and limit the rotation mechanically to 90 °?
I'm not aware of 90 ° potentiometers off the shelf.

Oh I'm not sure. Maybe I need to open up more about what this is all about. I bought a bidirectional DC-motor controller. I would like to use a potentiometer on the joystick. But the joystick can't turn it enough. If there was any trick to make this work?
Maybe this brings even more difficulty, but I would supply this controller with either 12vdc or 24vdc. Does it make this more difficult?

I tried to put a picture of the connection but it doesn't work :(

I hope you could help me with this problem.
 

bertus

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Nov 8, 2019
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Hello,

Why do you need a 90 degrees potmeter?
The circuit uses a standard 100 k potmeter:
bi_speed_contol.png
Bertus
 

bertus

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Hello,

You can adjust the range for the circuit with the 100k and 10k resistors on adjusting stack.
You could make the circuit and measure the voltage on pin 3 of IC1a.
Then you can calculate the needed resistors in stead of the 100k and 10k.

Bertus
 

Rockgirl

Aug 26, 2021
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Hello,

You can adjust the range for the circuit with the 100k and 10k resistors on adjusting stack.
You could make the circuit and measure the voltage on pin 3 of IC1a.
Then you can calculate the needed resistors in stead of the 100k and 10k.

Bertus
Oh I guess I don't know how to do that :(
 

Harald Kapp

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You're not alone in searching a pot with 90 ° rotation angle. All references I can find mention standard 270 ° pots with mechanically limited rotation angle. The ones that have "90 °" in the title of the sales offer refer to the angle between the pins and the potentiometer axle as 90 °, not the rotation angle.

In your circuit the pot is used as a voltage divider together with the 10 kΩ resistor at the lower end and the 100 kΩ resistor at the upper end. It thus delivers these voltages to pin 3 of IC1a:
0.05 × V+ when in the lower position,
0.52 × V+ when in the upper position
You cannot achieve this exact ratio by simple means. You can approximate it, however:
  • Remove the upper 100 kΩ resistor by a short circuit.
  • Change the lower 10 kΩ resistor to 5 kΩ (use 2 × 10 kΩ in parallel or use 4.7 kΩ, good enough)
  • Change the gain of IC1a from 1×to 2× by adding two resistors in the feedback as shown e.g. here. The value of the 2 additional resistors is not important. both shall have the same vale in the range 1 kΩ to 10 kΩ, whatever you have in your parts bin.
  • Mechanically limit the potentiometer to move from the left side (lower pin in the diagram) to left side + 90 °.
Using 4.7 k will give you these divider ratios:
0.1 × V+ in the lower position
0.72 × V+ in the upper position.
You can tweak (fine tune) these values by changing the gain f the opamp IC1a from 2 to another (lower) value. Do this by changing the resistor values in the feedback circuit according to the equations given in the link I providd above.
 

Rockgirl

Aug 26, 2021
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You're not alone in searching a pot with 90 ° rotation angle. All references I can find mention standard 270 ° pots with mechanically limited rotation angle. The ones that have "90 °" in the title of the sales offer refer to the angle between the pins and the potentiometer axle as 90 °, not the rotation angle.

In your circuit the pot is used as a voltage divider together with the 10 kΩ resistor at the lower end and the 100 kΩ resistor at the upper end. It thus delivers these voltages to pin 3 of IC1a:
0.05 × V+ when in the lower position,
0.52 × V+ when in the upper position
You cannot achieve this exact ratio by simple means. You can approximate it, however:
  • Remove the upper 100 kΩ resistor by a short circuit.
  • Change the lower 10 kΩ resistor to 5 kΩ (use 2 × 10 kΩ in parallel or use 4.7 kΩ, good enough)
  • Change the gain of IC1a from 1×to 2× by adding two resistors in the feedback as shown e.g. here. The value of the 2 additional resistors is not important. both shall have the same vale in the range 1 kΩ to 10 kΩ, whatever you have in your parts bin.
  • Mechanically limit the potentiometer to move from the left side (lower pin in the diagram) to left side + 90 °.
Using 4.7 k will give you these divider ratios:
0.1 × V+ in the lower position
0.72 × V+ in the upper position.
You can tweak (fine tune) these values by changing the gain f the opamp IC1a from 2 to another (lower) value. Do this by changing the resistor values in the feedback circuit according to the equations given in the link I providd above.
Hi, I would like to try this. Would it be possible for you to show (draw) how this is done for example in this picture you added?
I’m a girl and this isn’t my strongest skill, not yet at least :) I'm afraid if I destroy this controller by doing wrong :( I want to be sure what I do. I know how to solder and I know some basic stuff about electronics but I’m not sure about this.
 

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Rockgirl

Aug 26, 2021
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The solution was found closer than I could imagine. I found an old joystick potentiometer. It looks bigger compared to other potentiometer. I check those multimeter and I get 110k ohms. This potentiometer work those angle which I want. But why do these pots work differently? Are joystick pots some way different?

Another issue related to this bi-directional control. Is it a feature of this control that it does not start running the engine completely at zero? When I rotate the potentimeter the motor starts to buzz at the beginning and then it suddenly starts to rotate. I don’t mean it starts rotating at full, but about 3-4 volts. Why doesn't the motor run in the 1-3v range?
No matter which potentiometer is used for this, the potentiometer will not affect this. I would like this to work in the same way when I give power to the motor power supply unit, when I start from zero and add volts the motor starts a little quietly spinning up to 1 volts.
 
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